University of Basel  >  Condensed Matter Theory

Eugene Sukhorukov, Dr.

Curriculum Vitae

Noise of a Quantum-Dot System in the Cotunneling Regime
Eugene V. Sukhorukov, Guido Burkard, Daniel Loss
cond-mat/0010458

We study the noise of the cotunneling current through one or several tunnel-coupled quantum dots in the Coulomb blockade regime. The various regimes of weak and strong, elastic and inelastic cotunneling are analyzed for quantum-dot systems (QDS) with few-level, nearly-degenerate, and continuous electronic spectra. We find that in contrast to sequential tunneling where the noise is either Poissonian (due to uncorrelated tunneling events) or sub-Poissonian (suppressed by charge conservation on the QDS), the noise in inelastic cotunneling can be super-Poissonian due to switching between QDS states carrying currents of different strengths. In the case of weak cotunneling we prove a non-equilibrium fluctuation-dissipation theorem which leads to a universal expression for the noise-to-current ratio (Fano factor). In order to investigate strong cotunneling we develop a microscopic theory of cotunneling based on the density-operator formalism and using the projection operator technique. The master equation for the QDS and the expressions for current and noise in cotunneling in terms of the stationary state of the QDS are derived and applied to QDS with a nearly degenerate and continuous spectrum.

Andreev-Tunneling, Coulomb Blockade, and Resonant Transport of Non-Local Spin-Entangled Electrons
Patrik Recher, Eugene V. Sukhorukov, Daniel Loss
cond-mat/0009452

We propose and analyze a spin-entangler for electrons based on an s-wave superconductor coupled to two quantum dots each of which is tunnel-coupled to normal Fermi leads. We show that in the presence of a voltage bias and in the Coulomb blockade regime two correlated electrons provided by the Andreev process can coherently tunnel from the superconductor via different dots into different leads. The spin-singlet coming from the Cooper pair remains preserved in this process, and the setup provides a source of mobile and nonlocal spin-entangled electrons. The transport current is calculated and shown to be governed by a two-particle Breit-Wigner resonance which allows the injection of two spin-entangled electrons into different leads at exactly the same orbital energy, which is a crucial requirement for the detection of spin entanglement via noise measurements.The coherent tunneling of both electrons into the same lead is suppressed by the on-site Coulomb repulsion and/or the supercondcuting gap, while the tunneling into different leads is suppressed through the initial separation of the tunneling electrons. In the regime of interest the particle-hole excitations of the leads are shown to be negligible. The Aharonov-Bohm oscillations in the current are shown to contain single- and two-electron periods with amplitudes that both vanish with increasing Coulomb repulsion albeit differently fast.

Shot Noise by Quantum Scattering in Chaotic Cavities
S. Oberholzer, E.V. Sukhorukov, C. Strunk, C. Schonenberger, T. Heinzel, K. Ensslin, M. Holland
cond-mat/0009087

We have experimentally studied shot noise of chaotic cavities defined by two quantum point contacts in series. The cavity noise is determined as 1/4*2e|I| in agreement with theory and can be well distinguished from other contributions to noise generated at the contacts. Subsequently, we have found that cavity noise decreases if one of the contacts is further opened and reaches nearly zero for a highly asymmetric cavity.

Quantum Dot as Spin Filter and Spin Memory
Patrik Recher, Eugene V. Sukhorukov, Daniel Loss
Phys. Rev. Lett. 85 1962-1965 (2000); cond-mat/0003089

We consider a quantum dot in the Coulomb blockade regime weakly coupled to current leads and show that in the presence of a magnetic field the dot acts as an efficient spin-filter (at the single-spin level) which produces a spin-polarized current. Conversely, if the leads are fully spin-polarized the up or down state of the spin on the dot results in a large sequential or small cotunneling current, and thus, together with ESR techniques, the setup can be operated as a single-spin memory.

Quantum Computation and Spin Electronics
D. P. DiVincenzo, G. Burkard, D. Loss, and E. Sukhorukov.
See cond-mat/9911245.
Published in Quantum Mesoscopic Phenomena and Mesoscopic Devices in Microelectronics,
ed. I. O. Kulik (NATO Advanced Study Institute, Turkey, June 13-25, 1999).

In this chapter we explore the connection between mesoscopic physics and quantum computing.  After giving a bibliography providing a general introduction to the subject of quantum information processing, we review the various approaches that are being considered for the experimental implementation of quantum computing and quantum communication in atomic physics, quantum optics, nuclear magnetic resonance, superconductivity, and, especially, normal-electron solid state physics.  We discuss five criteria for the realization of a quantum computer and consider the implications that these criteria have for quantum computation using the spin states of single-electron quantum dots.  Finally, we consider the transport of quantum information via the motion of individual electrons in mesoscopic structures; specific transport and noise measurements in coupled quantum dot geometries for detecting and characterizing electron-state entanglement are analyzed.

Transport and Noise of Entangled Electrons
Eugene V. Sukhorukov, Daniel Loss, Guido Burkard
Proceedings of the XVI Sitges Conference (Lecture Notes in Physics, Springer), see cond-mat/9909348.

We consider a scattering set-up with an entangler and beam splitter where the current noise exhibits bunching behavior for electronic singlet states and antibunching behavior for triplet states. We show that the entanglement of two electrons in the double-dot can be detected in mesoscopic transport measurements. In the cotunneling regime the singlet and triplet states lead to phase-coherent current contributions of opposite signs and to Aharonov-Bohm and Berry phase oscillations in response to magnetic fields. We analyze the Fermi liquid effects in the transport of entangled electrons.

Quantum Computing and Quantum Communication with Electrons in Nanostructures
Daniel Loss, Guido Burkard, Eugene V. Sukhorukov.
Proceedings of the XXXIVth Rencontres de Moriond "Quantum Physics at Mesoscopic Scale", held in Les Arcs, Savoie, France, January 23-30, 1999; see cond-mat/9907133.

Addressing the feasibilty of quantum communication with electrons we consider entangled spin states of electrons in a double-dot  which is weakly coupled to in--and outgoing leads. We show that the entanglement of two electrons in the  double-dot  can be detected in mesoscopic transport and noise measurements. In the Coulomb blockade and cotunneling regime the singlet and triplet states lead to  phase-coherent current and noise contributions of opposite signs and to  Aharonov-Bohm and Berry phase oscillations in response to magnetic fields. These oscillations are a genuine two-particle effect and provide a direct measure of  non-locality  in  entangled states. We show that the ratio of zero-frequency noise to current (Fano factor) is universal and equal to the electron charge.

Probing Entanglement and Non-locality of Electrons in a Double-Dot via Transport and Noise
Daniel Loss and Eugene V. Sukhorukov.
Phys. Rev. Lett. 84 1035-1038 (2000); cond-mat/9907129.

Addressing the feasibilty of quantum communication with electrons we consider entangled spin states of electrons in a double-dot which is weakly coupled to in--and outgoing leads. We show that the entanglement of two electrons in the double-dot can be detected in mesoscopic transport and noise measurements. In the Coulomb blockade and cotunneling regime the singlet and triplet states lead to phase-coherent current and noise contributions of opposite signs and to Aharonov-Bohm and Berry phase oscillations in response to magnetic fields. These oscillations are a genuine two-particle effect and provide a direct measure of non-locality in entangled states. We show that the ratio of zero-frequency noise to current (Fano factor) is universal and equal to the electron charge.

Noise of Entangled Electrons: Bunching and Antibunching
Guido Burkard, Daniel Loss and Eugene V. Sukhorukov
Phys. Rev. B 61 16303-16306 (2000); cond-mat/9906071.

Addressing the feasibility of quantum communication with entangled electrons we consider a scattering set-up with an entangler and beam splitter where the current noise exhibits bunching behavior for electronic singlet states and antibunching behavior for triplet states. The Fano factor (noise-to-current ratio) is calculated and shown to contain opposite signs for singlets and triplets. We further show that spin currents can produce noise even in the absence of any charge currents.

Semi-classical Theory of Conductance and Noise in Open Chaotic Cavities
Ya. M. Blanter, and E. V. Sukhorukov
Phys. Rev. Lett. 84 1280-1283 (2000); see cond-mat/9904448.

Conductance and shot noise of an open cavity with diffusive boundary scattering are calculated within the Boltzmann-Langevin approach. In particular, conductance contains a non-universal geometric contribution, originating from the presence of open contacts. Subsequently, universal expressions for multi-terminal conductance and noise valid for all chaotic cavities are obtained classically basing on the fact that the distribution function in the cavity depends only on energy and using the principle of minimal correlations.

Noise in Multiterminal Diffusive Conductors: Universality, Nonlocality and Exchange Effects
Eugene V. Sukhorukov and Daniel Loss
Phys. Rev. Lett. 80, 4959 (1998), see cond-mat/9802050;
Phys. Rev. B 59, 13054-13066 (1999), see cond-mat/9809239.

We study noise and transport in multiterminal diffusive conductors. Using a Boltzmann-Langevin equation approach we reduce the calculation of shot-noise correlators to the solution of diffusion equations. Within this approach we prove the universality of shot noise in multiterminal diffusive conductors of arbitrary shape and dimension for purely elastic scattering as well as for hot electrons. We show that shot noise in multiterminal conductors is a non-local quantity and that exchange effects can occur in the absence of quantum phase coherence even at zero electron temperature. It is also shown that the exchange effect measured in one contact is always negative -- in agreement with the Pauli principle. We discuss a new phenomenon in which current noise is induced by thermal transport. We propose a possible experiment to measure locally the effective noise temperature. Concrete numbers for shot noise are given that can be tested experimentally.

Magnetic-field-dependent Zero-bias Diffusive Anomaly in Pb Oxide-n-InAs Structures: Coexistence of Two- and Three-dimensional States
G. M. Minkov, A. V. Germanenko, S. A. Negachev, O. E. Rut, Eugene V. Sukhorukov
Phys. Rev. B 59 13139-13146 (1999), see cond-mat/9807041.

The results of experimental and theoretical studies of zero-bias anomaly (ZBA) in the Pb-oxide-n-InAs tunnel structures in magnetic field up to 6T are presented. A specific feature of the structures is a coexistence of the 2D and 3D states at the Fermi energy near the semiconductor surface. The dependence of the measured ZBA amplitude on the strength and orientation of the applied magnetic field is in agreement with the proposed theoretical model. According to this model, electrons tunnel into 2D states, and move diffusively in the 2D layer, whereas the main contribution to the screening comes from 3D electrons.